Inorganic mesoporous solids are well known and the synthesis thereof, in particular via a surfactant structuring effect, was described for the first time in U.S. Pat. No. 3,556,725.
These mesoporous solids (or else mesoporous zeolites, or else zeolites with a mesoporous structure) are highly useful in many industrial fields, both as catalysts and catalyst supports, but also adsorbents, in so far as their large porosity expressed in terms of [surface area/volume] ratio allows the molecules with which they come into contact to readily access the core of the particles and to react on a large surface area, thus enhancing the catalytic and/or adsorbent properties of these materials.
The company Mobil, during the 1990s, undertook extensive studies relating to mesoporous inorganic solids, in particular relating to (alumino)silicic compounds, and more particularly the compound MCM 41 (for Mobil Composition Of Matter 41), for which a synthesis process is described in Nature, (1992), vol. 359, pp. 710-712, and which were the subject of numerous subsequent patents and scientific articles.
Thus, these mesoporous materials are now well known on the laboratory scale, both as regards their pore structure and distribution in their modes of synthesis, and as regards the possible applications thereof as catalysts and/or as adsorbents. However, these mesoporous inorganic materials have the major drawback of being thermally unstable in the presence of water, which greatly limits the industrial applications.
The search for mesoporous inorganic solids led to the development of mesoporous zeolites obtained by various processes, as described, for example, in the article by Feng-Shou Xiao et al, (Hierarchically Structured Porous Materials, (2012), 435-455, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany 978-3-527-32788-1).
One of the routes envisaged is that of post-treatments of zeolite crystals, it being possible for these post-treatments to be, for example, treatments with water vapour, acidic and/or basic treatments which result in dealumination, and additional treatments to remove extra-network species, it being possible for all these post-treatments to each or together be carried out one or more times, simultaneously or consecutively.
U.S. Pat. No. 8,486,369, patent applications US 2013 0183229, US 2013 0183231, and also application WO 2013/106816 are examples which illustrate such processes for preparing zeolite with a mesoporous structure by various successive post-treatments with vapour and then with acids in the presence of surfactant.
While such processes have a tendency to create large pore volumes, as a counterpart they greatly degrade the crystallinity of the initial zeolite powder, up to close to 50% in certain cases. It is, moreover, necessary to resort to additional cauterization treatments to stabilize the zeolite framework, to remove the extra-network aluminium atoms in order to make possible the subsequent heat treatments, in particular calcination treatments, required for most of the uses of zeolite materials as catalysts or adsorbents.
Such processes which make it possible to create a certain mesoporosity within zeolite solids are therefore very laborious to implement due to the succession of numerous steps, which are sparingly economical and therefore difficult to industrialize. In addition, the multitude of steps has a tendency to embrittle the zeolite structure and, consequently, to reduce the intrinsic properties of these zeolites.
This is why syntheses of mesoporous zeolite solids directly and without post-treatment known as such in the prior art are nowadays preferred. Various publications show the feasibility of the laboratory synthesis of mesoporous zeolites, and, by way of example, applications WO 2007/043731 and EP 2 592 049 are in particular noted, in which the synthesis of mesoporous zeolites is carried out based on surfactant, and in particular that of TPOAC ([3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride) type.
Yet other publications illustrate such studies, for instance those of R. Ryoo (Nature Materials, 5, (2006), 718 sqq.) which describe a synthesis of LTA with mesopores, or those of A. Inayat et al. (Angew. Chem. Int. Ed., (2012), 51, 1962-1965) which describe the synthesis of mesoporous FAU (X) using TPHAC ([3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride), as structuring agent.
However, there is not at the present time any description concerning the preparation of agglomerates based on mesoporous zeolites, in which agglomerates the specific properties of these mesoporous zeolites, in particular their microporosity, are conserved. As a result, there remains at the present time no industrial application, in particular in the field of the separation of liquids and/or of gases, of ion exchange or in the field of catalysis, using such zeolite agglomerates with high microporosity comprising at least one mesoporous zeolite and the transfer kinetics of which are at least comparable to those expected owing to the presence of the mesoporosity.
There therefore remains today a need for zeolite adsorbents which have a high microporosity, i.e. a large adsorption capacity, but also which allow optimized transfers, in particular through the presence of mesopores. Thus, the current need of companies is today moving towards zeolite adsorbents which ally both an optimum adsorption capacity and optimum transfer kinetics.
It should also be recalled that the industry, and in particular the fields of application mentioned above, uses zeolite adsorbents in agglomerate form. Indeed, synthetic zeolites are usually obtained after a process of nucleation and crystallization of silicoaluminate gels in which the size of the crystallites produced is from about one micrometre to a few micrometres: they are then referred to as crystals of zeolite or zeolite in powder form.
These powders are not easy to use industrially since they are difficult to manipulate on account of their poor flowability; they generate substantial losses of pressure, and also poor distribution of the streams in the beds, in particular in dynamic processes involving fluids in flow.
Consequently, agglomerated forms of these powders are preferred, which are more commonly referred to as zeolite agglomerates or agglomerated zeolite adsorbents and which may be in the form of grains, strands, extrudates or other agglomerates, these said forms possibly being obtained by extrusion, pelletizing, atomization or other agglomeration techniques that are well known to those skilled in the art. These agglomerates do not have the abovementioned drawbacks inherent in pulverulent materials.
These agglomerates generally consist of zeolite crystals and of a binder, which is usually inert with respect to the application for which the zeolite is intended, said binder being intended to provide the cohesion of the zeolite crystals with one another and to give them the sufficient and necessary mechanical strength for the industrial application envisaged.